Dams are solid barriers constructed across a river or a natural stream to create an artificial reservoir on its upstream side for storing water. It may also be constructed to facilitate diversion of water from the river or to retain debris flowing in the river
The first known dams were constructed about 8,000 years ago in Mesopotamia
Dams built in Jordan and Egypt – 1100BC
Dujiang Irrigation Project in China – 300BC – 800,000 hectares
Large Dams in the world (> 15 m high)
In 1949 some 5,000 Existed.
By 2000, there were 45,000.
With the exception of the Great Wall of China, dams are the largest structures ever built
Throughout history, big dams have prevented flooding, irrigated farmland, and generated tremendous amounts of electricity
Purpose of the Dams
Irrigation
Generation of Electrical Energy
Municipal and Industrial Water Supply
Navigation
Flood Control
Recreation
Diversion
Catching and retaining debris
Impoundment Facility
Dams in the World and its Roles
1. Tailings Dam, River Rimac, Peru, frequently contain extremely toxic materials
2. Cowlitz Falls Dam, Lewis River, Washington for Power Generation
Various types of dams are constructed based on their usage. These dams are classified based on construction material used, structure and design, functions, hydraulic design and size.
Classification Based on Construction Material used for a Dam
A dam is said to be Rigid Dam if it is constructed using rigid construction materials such as masonry, concrete, steel, timber etc. The basic shape of rigid dam is triangular. Rigid dams constructed using different rigid materials are discussed below.
Masonry dams are built using either stone masonry or brick masonry. Cement mortar is used to join the masonry blocks. Gravity dam, arch dam etc. are examples of masonry dams.
Concrete is most commonly used material to construct a dam. Most of the major dams in the world are built using concrete. Gravity dams, arch dam, Buttress dam etc. can be constructed using concrete.
Timber dams generally used for temporary purposes such as to divert the water for the construction of main dam, to control flood water flow etc. Timber dams are suitable up to 9 meters height.
Steel dams are also used for temporary requirements like timber dams. Steel plates and inclined struts are used for the construction of steel dam. This type of dams are suitable up to 15 to 18 meters of height.
Non-rigid dams are constructed using non rigid materials such as earth, rocks etc. Basic shape of non-rigid dam Trapezoidal. Non-rigid dams of different materials are
Earthen dams are made of ordinary soil which is cheaply available. This type of dams are suitable where the foundation soil is very weak and not strong enough to carry the weight of masonry dam. Since it is constructed using soil the cost of construction is very less compared to rigid type dam.
Fig 5: Earthen Dam
6. Rock Fill Dams
Rock fill dams are constructed using rocks and boulders. Upstream side of dam is built with dry rubble masonry and loose rock fill is provided on the downstream side. A reinforced concrete slab layer is also provided on the upstream side to make it water tight.
It is more stable than earthen dams and its flexible nature helps it better against earthquake forces.
A Gravity dam is a structure which resists the external forces by its own weight or self-weight. Gravity dams are generally constructed by using masonry or concrete.
Various external forces like water pressure, uplift pressure, wave pressure, ice pressure, earth quake pressure etc. are resisted by its self-weight only which acts vertically downwards. So, good foundation is required to construct gravity dam preferably rocky strata under the dam.
The shape of cross section of gravity dam is approximately triangular in shape. Infiltration gallery can be provided within the dam to resist uplift pressure. The failure of gravity dam may occurs due to sliding, overturning or crushing at toe. Hence, higher factor of safety is recommended for the design of gravity dam.
An arch dam is curved in plan with its convex upstream. Various forces coming onto the dam are resisted by its arch action. It is constructed using masonry or concrete but requires less material compared to gravity dam.
The loads coming onto the dam are transferred to the abutments of dam. So, abutments must be stronger and generally natural formations like hills are used as abutments. Arch dams are generally preferred for narrow valleys.
Arch dam is economical when the length of dam is less than its height. So, this type of dams can be build up to greater heights.
A buttress dam contains face slab, buttresses and base slab. Face slab is provided on the upstream side with some inclination and this slab is supported by series of buttresses which are nothing but supports. Base slab acts as foundation for the whole dam which receives the load from buttresses and face slab.
Buttress dam is either straight or curved in plan. Greater the height of dam higher the number of buttresses. This type of dams are preferred where the foundation soil is very weak. The space available between the buttresses can used for several purposes like water treatment plant installation, power plant installation etc.
Embank dam is made of soil or rocks. This type of dams are come under non rigid type dams. Embankment dams are again classified into three different types
If the dam is constructed using only one type of soil then it is called as homogeneous embankment type dam. But homogeneity of soil makes the dam pervious and allow seepage of water through the dam. To overcome this stone pitching is recommended on the upstream side.
Zoned embankment dam consists an impervious soil zone inside the pervious soil layer. Clay or silt or mixture of clay and silt is used to make the impervious zone. Ordinary soil is used for make the pervious outer layer.
Diaphragm embankment dam consists a diaphragm inside the earthen dam and it is made of impervious soils or concrete or steel or timber. This diaphragm prevents the seepage of water through dam section.
Storage dam is constructed to store water on the upstream side especially during rainy seasons and is released during dry weather season or when there is higher demand of water. The stored water can also be used to generate power, irrigation, water supply etc.
Diversion dam is a dam which is constructed to divert the flow of water into other channel or canal. This type dams are generally used to fill the irrigation channels.
The main purpose of Detention dam is to control floods. During flood period, it stores the water and protects the downstream side from damage due to floods. After the flood period the stored water is released at a controlled rate without effecting the downstream side.
14. Debris Dam
Debris dam is built to retain the debris of the river water. Sand, gravel, driftwood etc. are debris generally flow with river water. The water on the downstream side of debris dam is clear.
Cofferdam is temporary structure which generally acts like diversion dam. Cofferdam provides dry area for the construction of main dam by diverting the water flow into other watercourse. It is constructed on upstream side or fully around the construction site.
Fig 16: Coffer Dam
16. Hydro-power Dam
Hydro-power dam is used to generate electricity by rotating turbines with the help of water falling from upstream side to downstream side of dam.
Fig 17: Cheruthoni Hydropower Dam, India
Classification Based on Hydraulic Design of Dam
Based on hydraulic design dams are classified into 2 types as follows
Overflow dam is a dam which allows the water over its crest when surplus water is flowing on the upstream side. This is also called as spillway dam or over fall dam.
Fig 18: Canyon Lake Dam, USA
18. Non-Overflow Dam
A dam which do not allow the surplus water to overflow over its crest. But in general spillway is provided for any type dam to release the excess water from upstream side.
Fig 19: Non-overflow Dam
Classification Based on Gross Storage of Dam
Dams are classified into three types based on gross storage capacity of dam and they are
Small Dam
Medium Dam
Large Dam
19. Small Dam
A dam is called as small dam if its gross storage capacity is in between 0.5 to 10 MCM (million cubic meters). Hydraulic head of small dam is generally about 7.5 to 12 meters.
Fig 20: Small Dam
20. Medium Dam
If gross storage capacity of a dam is in between 10 to 60 MCM then it is said to be medium storage dam. Its hydraulic head is 12 to 30 meters.
Fig 21: Medium Dam
21. Large Dam
A dam is said to be large dam if its gross storage capacity is above 60 MCM. Hydraulic head of a large dam is greater than 30 meters.
